Flutter SearchDelegates BuildResults isn'a a valid override

I'm trying to build a networkbased search. It works so far, that I get the results I want from my API. Now I want to build the results but get two error messages I don't know how to resolve.

Error 1:

The body might complete normally, causing 'null' to be returned, but the return type, 'FutureOr', is a potentially non-nullable type.\nTry adding either a return or a throw statement at the end.

Error 2:

FoodSearch.buildResults' ('Future Function(BuildContext)') isn't a valid override of 'SearchDelegate.buildResults' ('Widget Function(BuildContext)').

My buildResult:

@override
  Future<Widget> buildResults(BuildContext context) async {
    var split_kat = query.split('|');
    String fumiID = split_kat[2];
    FutureBuilder<FutterModell>(
      future: FumiFibelAPI.getSingleFood(fumiID),
      builder: (context, snapshot) {
        switch (snapshot.connectionState) {
          case ConnectionState.waiting:
            return Center(child: CircularProgressIndicator());
          default:
            if (snapshot.hasError) {
              return Container(
                color: Colors.black,
                alignment: Alignment.center,
                child: Text(
                  'Something went wrong!',
                  style: TextStyle(fontSize: 28, color: Colors.white),
                ),
              );
            } else {
              return Container();
            }
        }
      },
    );
  }

The API for the Future-List:

static Future<FutterModell> getSingleFood(String futtermittelID) async {
    const storage = FlutterSecureStorage();
    var token = await storage.read(key: "_authToken");
    int futtermittel = int.parse(futtermittelID);

    var url = Uri.parse(开发者_开发知识库'${Constants.BASE_URL}/futtermittel/$futtermittel');

    var headers = {
      'Content-Type': 'application/json',
      'Accept': 'application/json',
      'Authorization': 'Bearer $token',
    };

    var body = {
      "futtermittel_id": futtermittel,
    };

    var res = await http.post(url, headers: headers, body: jsonEncode(body));
    final result = json.decode(res.body);
    return result.map<FutterModell>(FutterModell.fromJson).toList();
  }

Thanks for the help!